linux-kernel - Re: [PATCH mm-unstable] mm: rmap: abstract updating per-node and per-memcg stats

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Date: Tue, 7 May 2024 10:52:24 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable] mm: rmap: abstract updating per-node and
 per-memcg stats

On 06.05.24 23:13, Yosry Ahmed wrote:
> A lot of intricacies go into updating the stats when adding or removing
> mappings: which stat index to use and which function. Abstract this away
> into a new static helper in rmap.c, __folio_mod_stat().
> 
> This adds an unnecessary call to folio_test_anon() in
> __folio_add_anon_rmap() and __folio_add_file_rmap(). However, the folio
> struct should already be in the cache at this point, so it shouldn't
> cause any noticeable overhead.

Depending on the inlining, we might have more branches that could be avoided
(especially in folio_add_new_anon_rmap()).

[the rmap code is more performance-sensitive and relevant than you might think]

> 
> No functional change intended.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
> 
> This applies on top of "mm: do not update memcg stats for
> NR_{FILE/SHMEM}_PMDMAPPED":
> https://lore.kernel.org/lkml/20240506192924.271999-1-yosryahmed@google.com/
> 
> David, I was on the fence about adding a Suggested-by here. You did
> suggest adding a helper, but the one with the extra folio_test_anon()
> was my idea and I didn't want to blame it on you. So I'll leave this up
> to you :)

:) fair enough! It's a clear improvement to readability.

[...]
>   
> -	if (nr_pmdmapped) {
> -		/* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
> -		if (folio_test_anon(folio))
> -			__lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
> -		else
> -			__mod_node_page_state(pgdat,
> -					folio_test_swapbacked(folio) ?
> -					NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED,
> -					-nr_pmdmapped);
> -	}
>   	if (nr) {
> -		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
> -		__lruvec_stat_mod_folio(folio, idx, -nr);
> -


We can now even do:

diff --git a/mm/rmap.c b/mm/rmap.c
index 9ed995da4709..7a147195e512 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1555,18 +1555,17 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
                 break;
         }
  
-       if (nr) {
-               /*
-                * Queue anon large folio for deferred split if at least one
-                * page of the folio is unmapped and at least one page
-                * is still mapped.
-                *
-                * Check partially_mapped first to ensure it is a large folio.
-                */
-               if (folio_test_anon(folio) && partially_mapped &&
-                   list_empty(&folio->_deferred_list))
-                       deferred_split_folio(folio);
-       }
+       /*
+        * Queue anon large folio for deferred split if at least one
+        * page of the folio is unmapped and at least one page
+        * is still mapped.
+        *
+        * Check partially_mapped first to ensure it is a large folio.
+        */
+       if (folio_test_anon(folio) && partially_mapped &&
+           list_empty(&folio->_deferred_list))
+               deferred_split_folio(folio);
+
         __folio_mod_stat(folio, nr, nr_pmdmapped);
  
         /*


Which will help some of my upcoming patches.

Feel free to include that in a v2, otherwise I'll include it in an upcoming
patch series.


Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb